1. Field of the Invention
The present invention relates generally to a fuel injection pump and, more particularly, to a fuel injection pump having a variable prestroke mechanism.
2. Description of the Related Art
Fuel injection pumps having a variable prestroke mechanism are known from Japanese Utility Model Kokai-Publication No. 58(1983)-114875, for example. The fuel injection pump of that example is illustrated in FIGS. 16 to 18.
Referring to FIGS. 16 to 18, a pump body 1 is fixedly provided with a plunger barrel 3 into which a plunger 4 is slidably inserted. That plunger 4 is designed to reciprocate upon receiving input from an engine. A portion of the plunger 4 facing a fuel reservoir chamber 18 is provided with a hole 24 through which an amount of fuel is sucked or discharged, and a control inclined groove 26. A control sleeve 17 is slidably fitted thereover and has a cut-off hole 28 extending therein in the radial direction. A control rod 29 is provided at right angles to the control sleeve 17. An engaging portion 36 extending radially from the control rod 29 is in engagement within an associated groove 22 formed in the control sleeve 17. Thus, turning of the control rod 29 causes turning of the engaging portion 36 in unison therewith. Turning of the portion 36 then causes vertical movement of the control sleeve 17, so that there is a variation in the vertical position of the plunger 4 relative to the control sleeve 17. This makes it possible to control the prestroke of plunger 4 which is the distance between the control sleeve 17 and the aforesaid hole 24.
When supplying fuel, the plunger 4 ascends to cause the hole 24 to be closed by the control rod 17 for the initiation of fuel injection (pumping of fuel), as illustrated in FIG. 16, while the injection of fuel ceases upon the control inclined groove 26 in communication with the hole 24 coming in communication with the cut-off hole 28 in the control sleeve 17, as shown in FIG. 17.
In some cases, however, as the plunger 4 continues to move upwardly even after the pumping of fuel has been finished, the control inclined groove 26 passes beyond the cut-off hole 28 and is closed by the control sleeve 17, as illustrated in FIG. 18. In this case, the upper portion of the groove 26 projects upwardly from the control sleeve 17, and communicates with the fuel reservoir chamber 18. However, especially when the prestroke is increased and the control sleeve 17 is close to a lower face 5a of a valve housing 5, an escape passage for high-pressure is very narrow so that the discharge of return fuel is restricted which causes a secondary injection of fuel due to a temporary increase in the pressure prevailing in an injection pipe.
Control of the prestroke, as mentioned above, leads to a variation in the range in which a cam is used to reciprocate the plunger 4. Thus, where a non-uniform speed cam is used, the rate of fuel injection relative to the speed of plunger 4 varies. As the rate of injection increases, the pressure of injection increases, so that, when the number of revolutions of an engine is increased, there is a possibility that the pressure of injection will exceed the allowable pressure provided for a fuel-pressurizing chamber or the injection pipe. A solution to this anticipated problem may be achieved by increasing the strength of the fuel injection pump. However, this results in an increase in the weight of the pump.